Tap changer monitoring

ABSTRACT

A method and means for diagnosing transformer tap changers is provided. Pressure is monitored in an insulating liquid of a tap changer for a period of time in which switching of the tap changer is carried through. Switching-related information is then extracted from the pressure diagram. The method, preferably used for single-phase tap changers, comprises the step of relating quantities from pressure peaks to specific arcings of the switching sequence. Early error detection is achieved, whereby transformed failures may be avoided. Repeated measurements ae followed by trend evaluation. If a trend indicating tap changer problems is detected, an alarm signal is triggered. The invention may include switching off the transformer if the pressure exceeds a predetermined emergency value as well as extracting information of the base pressure in the tap changer and converting it to level of insulating liquid.

TECHNICAL FIELD

The present invention relates in general to tap changers and inparticular to a method and apparatus for diagnosing the condition of atap changer.

BACKGROUND

High voltage and medium voltage transformers are widely used inelectrical power distribution of today. Utilizing the magnetic featuresof electrical currents, they transfer power between two or moreincompatible electrical AC-circuits. Thereby, power from a power plantcan be transported by a small current of very high voltage and thenstepped down to a large current of low voltage before reaching theusers.

Supply authorities are under obligation to their customers to maintainthe supply voltage between certain limits. A tap changer is a deviceused in a transformer for regulation of the transformer output voltagewithin these limits. Normally, this is achieved by changing the ratiosof the transformers of the system by altering the number of turns in onewinding of the appropriate transformer(s). A lower load on the systemmay for instance require that tap-changing operations decrease thenumber of turns in the winding. This ultimately results in an increasedoutput voltage as compared to if no tap changing were performed.Normally, a tap changer performs in the range of 5-20 changingoperations per day, but for very demanding systems, such as meltingfurnaces, there may be hundreds of such operations per day.

Besides the described application, tap changers may also be used inconnection with other inductive power devices such as reactors. Tapchangers are either on-load, i.e. operating while the transformer isenergized, or off-load and there is a wide range of models available. Atap changer generally comprises a number of switches for tap changingand a number of resistors or other impedances to preventshort-circuiting. Furthermore, the tap changer typically is filled withan insulating liquid, such as oil, which besides insulation offerscooling of the device.

Problems during switching in tap changers are not usual, but can be veryserious when they occur. Such problems are mostly related to a longswitching time, in turn a result of e.g. wear or broken details. A majordisadvantage is that the switching problems generally are not discoveredbefore the tap changer becomes entirely unusable. Often the wholetransformer has to be switched off in an emergency manner as aconsequence of inadequate tap changing, which implies considerableinconveniences for the consumers as well as extremely high expenses forthe power industry.

Inspection of a tap changer generally requires opening of the enclosureand removal of the switching mechanism from the oil. This causes alengthy service interruption and is hence performed as seldom aspossible. Thus, solving the error detection problem by tap changerinspections on a regular basis is not a desirable option.

It is known in the prior art to measure acoustic signals in tap changersto obtain information about the functional qualities thereof. Accordingto U.S. Pat. No. 5,492,014 acoustic signals resulting from, among otherthings, tap-changing operations are measured. The “sound” of theoperations is recorded and unusual sounds are used as indications ofunusual events. Such acoustic measurements require specially adaptedequipment and are often associated with problems due to resonance,spurious signals, disturbances, etc. Moreover, diagrams resulting fromacoustic measurements are generally very difficult to interpret, inparticular since every system has its own acoustic characteristics.

Accordingly, there is a strong demand for a method and means, whichfacilitates early detection of switching problems in a tap changer.

SUMMARY

A general object of the present invention is to improve diagnosing anderror detection in tap changers. A specific object is to provide adiagnosing method and means for early detection of switching-relatedproblems in a tap changer. Another object is to provide measuring meansfor early switching-error detection which are easy to implement.

These objects are achieved in accordance with the attached claims.

The present invention is based on the recognition that information aboutswitching operations in a tap changer can be obtained from pressuremonitoring of the tap changer insulating liquid. A diagnosing method isprovided comprising registering pressure in an insulating liquid of atap changer as a function of time for a period of time in which aswitching action of the tap changer is carried through, and extractingswitching-related information from the registered pressure diagram. Themethod is preferably used for a single-phase tap changer and comprisesthe step of relating quantities derived from the measured pressure peaksto specific arcings of the switching action. In this way, early errordetection is achieved, whereby transformer failures can be avoided.Repeated measurements at different loads are preferably performed,followed by trend evaluation. If a trend indicating tap changer problemsis detected, an alarm signal is triggered to call for a pre-scheduledtap changer inspection. The method according to the invention mayfurther comprise switching off the transformer if the pressure exceeds apre-determined value as well as extracting information of the basepressure in the tap changer and relating it to the level of insulatingliquid. These features are very advantageous, since they replaceconventional pressure guards and oil level sensors.

Another aspect of the present invention is to provide diagnosing meansfor a tap changer. Still other aspects of the invention are to provide atap changer, a transformer comprising a tap changer and a computerprogram product for pressure monitoring and evaluation of tap changerconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a transformer with a tap changersystem which may be used with the present invention;

FIG. 2 is a schematic view of an on-load tap changer which may be usedwith the present invention;

FIG. 3A-3F are circuit diagrams illustrating a switching sequence in thetap changer of FIG. 2;

FIG. 4 is an exemplary diagram of pressure versus time obtained inaccordance with the present invention;

FIG. 5 illustrates a measurement set-up in accordance with the presentinvention; and

FIG. 6 is a flow diagram of a method of tap changer diagnosing inaccordance with the present invention.

DETAILED DESCRIPTION

Throughout the drawings the same reference numbers are used for similaror corresponding objects.

FIG. 1 is a schematic illustration of a transformer with a tap changersystem which may be used with the present invention. A transformer tank10 comprising a tap changer 12 is shown. The illustrated tap changer 12is suspended from a transformer cover 14, but other tap changers 12 maybe arranged outside the transformer tank 10. Both the transformer tank10 and the tap changer 12 are filled with an insulating liquid,preferably oil, stored in an oil conservator 16. To avoid contaminationof transformer oil, e.g. from arcing which will be described withreference to FIG. 3, the tap changer 12 has a tight housing separatingits insulating liquid from the transformer insulating liquid. Power tooperate the tap changer 12 is supplied from a motor-drive mechanism 18,which is mounted on the outside of the transformer tank 10. The power istransmitted by means of shafts 20 and bevel gears 22.

FIG. 2 is a schematic view of an on-load tap changer, which may be usedwith the present invention. The illustrated tap changer 12 is of an UCtype and detailed information thereof can be found in our technicalguide “On-load tap changers, type UC” (available on the internet). Thetap changer 12 is formed of two main parts, a diverter switch 24 and atap selector 26, interrelated by connections 30. A conventional tophousing 28 of the diverter switch 24 comprises a three-way valve ontowhich, besides connection flanges and connections for test equipment, apressure guard (not shown) is mounted. The pressure guard trips a maincircuit breaker of the transformer if a certain pressure level isexceeded. Thus, the complete transformer is switched off in the event ofover-pressure and high oil flow rates in the conservator (16 in FIG. 1).

Basically, the tap selector 26 selects the desired tap upon initiationof a tap change. The diverter switch 24 then transfers the current fromthe tap in service to the selected tap. While this quick changeover isoccurring, a resistance is cut into the circuit to ensure that there isno break in transfer of current. This also limits the circulatingcurrent between the two taps of the winding during the switchingoperation.

A switching sequence in the tap changer of FIG. 2 will now be describedwith reference to the circuit diagrams of FIG. 3A-3F. The circuitincludes an untapped winding 32, a tapped winding 34 with taps 36 a, 36b, 36 c, tap selector contacts 37 a, 37 b, transition resistors 38 a, 38b and the diverter switch 24 with main contacts 39 a, 39 d andtransition contacts 39 b, 39 c. A duplicate circuit is provided in orderto make one circuit carry the load current, while switching is carriedout by the other. In FIG. 3A, the main contact 39 a of the diverterswitch 24 is closed, whereby the load current is allowed to bypass theresistor 38 a and is carried directly to the selector contact 37 a andtap 36 a of the tapped winding 34. The selector contact 37 b lies on afirst tap 36 b in the part of the circuit presently carrying no current(to the right in FIG. 3A). Still in a non-current state, selectorcontact 37 b switches to a selected second tap 36 c, resulting in FIG.3B. Thereafter, the diverter switch 24 rotates clockwise making the maincontact 39 a break in such way that the current now flows via thetransition contact 39 b and the transition resistor 38 a (FIG. 3C). Thebreaking occurs in the first current zero after contact separation.

In the next step of the switching sequence, illustrated in FIG. 3D, thediverter switch 24 has continued to rotate clockwise. The transitioncontact 39 c of the previously non-conducting circuit part is closedwhereafter the load current is divided into two branches and acirculating current flows in the circuit between the taps 36 a and 36 c.The circulating current is limited by the two transition resistors 38 a,38 b avoiding short-circuiting. When the transition contact 39 b in thenext stage (FIG. 3E) breaks, the entire load current flows via thetransition resistor 38 b from the selected tap 36 c. Finally, in FIG.3F, the switching sequence and thus the tap changing is completed withbypassing of the resistor 38 b by connection of the main contact 39 dand a full load current to tap 36 c is obtained. The number of windingsin the transformer and hence the voltage output level has been altered.A complete tap changing sequence of the described type typically lastsabout 50 ms.

The present invention is not limited to the described tap changercircuit. Other embodiments may for instance use change-over selectors toachieve a reduced number of turns in the windings as compared to thedescribed linear switching. Moreover, the number of resistors may varyand the resistors may be replaced by other impedances. The illustratedflag cycle operation may also be replaced by pennant cycle operation.

In the described embodiment there are thus two contact breaks in thediverter switch 24 during the described switching sequence, leading tothe situations of FIGS. 3C and 3E, respectively. As the contacts break,the high voltage give rise to arcing indicated by flash-symbols in FIGS.3C and 3E. In a successful switching operation, the life of an arc iscompleted within one half-cycle (max 10 ms at 50 Hz). The arcing causesthermal degradation of the insulating liquid, resulting in formation ofvolumes filled with gas. One consequence of this is that the gasformation in turn leads to sudden pressure changes in the insulatingliquid. Since the insulating liquid has a limited compressibility andthe casing is rigid, the pressure increase is distributed within theliquid and can be measured at almost any position. Another consequenceof the thermal degradation is that the insulating liquid iscontaminated.

The present invention is based on the recognition that information aboutswitching operations in a tap changer can be obtained from pressuremonitoring of the tap changer insulating liquid. As will be described indetail in the following, it has unexpectedly shown that pressurediagrams present a time resolution enough to identify different arcs andare therefore highly suitable as basis for extraction ofswitching-related information.

FIG. 4 is an exemplary diagram of pressure versus time obtained inaccordance with the present invention. Pressure was measured in theinsulating liquid of a single-phase tap changer for a period of timewell exceeding the tap changer switching time. The two arcs of theswitching sequence (compare FIGS. 3C and 3E) give rise to two verydistinct pressure peaks. The first pressure peak originates from the arccaused by breaking the main breaking contact, while the second peak isrelated to breaking of the transition contact. After the second arcing,and the associated pressure peak, the pressure in the tap changerfluctuates with lower amplitude and the mean pressure slowly decreasesuntil the base pressure P₀ is regained. (FIG. 4 does not cover a timeperiod long enough for this to be fully visible.)

Since the pressure peaks indisputably can be related to a respectivearcing, features of the peaks, such as height, width and separationtime, provides information of the arcing and thereby of the switchcondition in the tap changer. Exemplary conclusions drawn from pressurediagrams of the type disclosed in FIG. 4 will now be given.

The intensity, i.e. a height or area measure, of a pressure peak ismainly related to the arc power. A high pressure peak implies fastformation of relatively large gas bubbles. This is due to a highelectrical current and/or a relatively long arcing time. However, thearcing time may not exceed one half-cycle to manifest itself as a highpressure peak. If the arcing time, on the other hand, does exceed half aperiod, a relighted arc may be present, e.g. due to minor switchingproblems or overload. Since this leads to a relatively long duration ofgas formation, it can be identified as a wide pressure peak. Thus, thewidth of a pressure peak generally provides information of the durationof the corresponding arc.

Evaluation of the separation in time of two pressure peaks, e.g. onefrom a main contact and one from a transition contact, provides veryuseful information about the tap changer switching speed. A relativelylong period of time between the peaks indicates a low switching speed,or in other words a poor breaking ability. This could even result in arelighted arc implying wide peaks. Furthermore, the time between arcscorrespond to well defined specifications of the tap changer switches.By comparing the time in the pressure diagram with the specifications, adeviation may be detected. Such a deviation indicates severe switchingproblems in the tap changer.

Finally, information about the status of the tap changer may also beretained from the time until the pressure reaches the base pressure P₀following a switching sequence. A very long such decay time could becaused by a non-working connection to the expansion tank. Generally,this would also manifest itself as larger amplitudes of the oscillationsfollowing the transition contact peak.

It should be noted that the above-described diagnosing and errordetection situations are merely given as examples. Of course, otherswitching-related information may as well be interpreted from a pressurediagram according to the present invention. Furthermore, the inventioncovers tap changer systems with different numbers of contact breaks,i.e. different arcing situations leading to other diagramcharacteristics. Embodiments with pennant cycle operation would forexample disclose a different number of arcs and consequently a differentnumber of peaks in the pressure diagram.

Preferably, diagnosing based on pressure monitoring according to thepresent invention is used for single-phase tap changers. Single-phasetap changers imply pressure diagrams similar to FIG. 4, in which thepressure peaks are clearly distinguishable and from which switchinginformation hence is easy to extract. If the same measurement wasperformed for a multi-phase tap changer, where each phase implies asimilar switching sequence as in FIG. 3, there would be two additionalpressure peaks for each additional phase and a time delay between thepairs of peaks. The result could be overlapping pressure peaks, makingthe diagram interpretation somewhat more complicated. The inventioncovers tap changers of any number of phases, i.e. triple or othermultiple-phase systems as well as single-phase tap changers.Nevertheless, it is evident that a single-phase tap changer is a systemwhere the present invention is easiest to implement. Most favourable isto use the invention for tap changer systems with rigid devices andcasings, since the peaks in the pressure diagram then become very sharp.

The pressure monitoring and diagram evaluation according to the presentinvention is preferably performed continuously or repeated at suitabletime intervals. Then, pressure peaks formed at different loads on thetap changer system are registered. In this way, identification of trendsrelated to switching features is possible. One embodiment of theinvention comprises triggering of an alarm signal if e.g. an increasingor decreasing trend in the height or width of the pressure peaks isdetected. The alarm signal implies that a pre-scheduled inspection ofthe tap changer should be performed whenever suitable.

The major benefit of the present invention is that it enables earlydetection of switching related errors. In the above-described way,incipient problems related to the tap changing function may beidentified before they cause major damages. This implies that theoccurrence of transformer failures decreases, which in turn is veryadvantageous for economical as well as environmental reasons.

There are additional advantages associated with embodiments of theinvention. Firstly, the pressure monitoring according to the inventionmay be used to trig the pressure guard, i.e. to switch off thetransformer when a pre-determined emergency pressure value is exceeded.The pressure diagram can be useful in order to determine whether the tapchanger pressure really is increasing in a long-term point-of-view or ifthe pressure merely has risen instantaneously following a comparativelystrong arcing. As a result, transformer failures due to “false” pressurealarms may be avoided, reducing the expenses of the power industry.Secondly, the base pressure of the pressure diagrams obtained inaccordance with the present invention may be extracted and converted tothe level of insulating liquid (generally oil) in the tap changer. Inthis way, no separate oil level sensor is needed in the tap changer.

FIG. 5 illustrates a measurement set-up in accordance with the presentinvention. The transformer tank 10, into which the tap changer 12 issuspended, is shown. A pressure sensor 40 measures the pressure of theinsulating liquid in the tap changer 12. Preferably, a very fastpressure sensor 40 is used in order to properly register changes in theinsulating liquid pressure during a tap changing sequence. For themeasurements resulting in the diagram of FIG. 4 a pressure sensor oftype DPI 260 available from Druck Ltd, with a response of 2 kHz, wasused. The pressure sensor 40 is in a preferred embodiment of theinvention arranged in the top housing (28 in FIG. 2) of the diverterswitch. Existing connections may then be employed and a simple solutionfor pressure sensor implementation is achieved. This constitutes yetanother advantage of the present invention.

The pressure signal is transferred from the pressure sensor 40 to means42 for signal storing. From there, evaluation means 44 may collect datafor diagnosing and evaluation for instance according to theabove-described principles. When a trend is detected, the evaluationmeans 44 makes alarm-triggering means 46 release an alarm signalnotifying an operator 48 of the need for tap changer inspection. Such aninspection typically involves both the transformer and the tap changer12. The evaluation means also communicates with means 50 for switchingoff the transformer. For the case of (non-false) over-pressure, theswitch off-means 50 turns off the transformer. The actual hardware ofthe described means 42, 44, 46, 50 is conventional as such and will notbe described further herein, since it is not in itself constituting apart of the present invention. However, the use thereof in thisparticular arrangement is new.

FIG. 6 is a flow diagram of a method of tap changer diagnosing inaccordance with the present invention. In step S1, a switching action ofa tap changer is performed. The pressure in the insulating liquid of thetap changer is registered as a function of time in step S2. In step S3,it is examined whether the tap changer pressure exceeds a pre-determinedemergency level and preferably controlled that the change is not causedby known normal switching operations. If so is the case, theover-pressure results in switching off the transformer in step S4. If noover-pressure is at hand, pressure diagrams can be used to extractswitching-related information in step S5. Furthermore, the base pressuremay be registered and converted to level of insulating liquid in stepS6. Preferably, the pressure measuring is repeated for a time periodcovering numerous switching actions at different loads on the tapchanger. When several pressure diagrams have been recorded, trends inswitching-related information can be evaluated in step S7. Step S8checks whether there is an increasing or decreasing trend inswitching-related information. Of course this involves determining howlarge a deviation has to be to imply a significant trend. If there is asignificant trend, an alarm signal is triggered in step S9, whereafterthe tap changer preferably is inspected in step S10. Otherwise, thediagnosing method steps are repeated beginning with the switching actionin the step S1.

The method according to the present invention may be implemented bymeans of software, hardware, or a combination thereof. A computerprogram product implementing the method or a part thereof comprises asoftware or a computer program run on a general purpose or speciallyadapted computer, processor or microprocessor. The software includescomputer program code elements or software code portions that make thecomputer perform the method using at least one of the steps previouslydescribed in FIG. 6. The program may be stored in whole or part, on, orin, one or more suitable computer readable media or data storage meanssuch as a magnetic disk, CD-ROM or DVD disk, hard disk, magneto-opticalmemory storage means, in RAM or volatile memory, in ROM or flash memory,as firmware, or on a data server.

Although the invention has been described herein with reference tospecific illustrated embodiments thereof, it should be emphasized thatthe invention also covers equivalents to the disclosed features, as wellas modifications and variants thereof that are obvious to the manskilled in the art. Therefore, the scope of the invention should only belimited by the enclosed claims.

1. Diagnosing method for a tap changer 4 having an insulating liquidcomprising the steps of performing a switching action of said tapchanger; and registering pressure in said insulating liquid as afunction of time for a period of time comprising the time in which saidswitching action is carried through, characterized by the further stepof extracting switching-related information from said registeredpressure function, said extracting step in turn comprising the steps ofidentifying a first pressure peak in said registered pressure function;relating quantities from said first pressure peak to quantities of afirst arcing of said switching action; relating said quantities of saidfirst arcing to breaking a first contact of a diverter switch in saidtap changer during said switching action; and diagnosing a switchingcondition associated with said first contact, the switching conditionbeing an error condition when quantities are outside a predeterminedinterval.
 2. The method according to claim 1, characterized in that saidextracting step further comprises the steps of identifying a secondpressure peak in said registered pressure function; relating quantitiesfrom said second pressure peak to quantities of a second arcing of saidswitching action; relating said quantities of said second arcing tobreaking a second contact of said diverter switch in said tap changerduring said switching action; and diagnosing a switching conditionassociated with said second contact, the switching condition being anerror condition when quantities are outside a predetermined interval. 3.The method according to claim 2, characterized in that said firstcontact is a main contact of said diverter switch in said tap changerand said second contact is a transition contact of said diverter switch.4. The method according to claim 2 or 3, characterized in that saidextracting step further comprises the step of relating the timeseparation between said first and second pressure peaks to switchingspeed.
 5. The method according to claim 1, characterized in that saidextracting step further comprises the step of relating a width of saidfirst and/or second pressure peak to arc duration.
 6. The methodaccording to claim 1, characterized in that said extracting step furthercomprises the step of relating an intensity measure of said first and/orsecond pressure peak to arc power.
 7. The method according to claim 1,characterized in that said tap changer is a single-phase tap changer. 8.The method according to claim 1, characterized by the further steps ofextracting information of a base pressure in said tap changer; andconverting said base pressure to a level of insulating liquid in saidtap changer.
 9. The method according to claim 1, characterized in thatsaid registering step is repeated, and by the further step of evaluatingtrends in said switching-related information.
 10. The method accordingto claim 9, characterized by the further step of triggering an alarmsignal if an increasing or decreasing trend in said switching-relatedinformation is detected.
 11. The method according to claim 1,characterized in that said insulating liquid is oil.
 12. Diagnosingmeans for a tap changer having an insulating liquid comprising means forregistering pressure in said insulating liquid as a function of time fora period of time comprising the time in which a switching action of saidtap changer is carried through, characterized by means for extractingswitching-related information from said registered pressure functioncomprising means for identifying a first pressure peak in saidregistered pressure function; means for relating quantities from saidfirst pressure peak to quantities of a first arcing of said switchingaction; means for relating said quantities of said first arcing tobreaking a first contact of a diverter switch in said tap changer duringsaid switching action; and means for diagnosing a switching conditionassociated with said first contact, the switching condition being anerror condition when quantities are outside a predetermined interval.13. The means according to claim 12, characterized in that said tapchanger is a single-phase tap changer.
 14. The means according to claim12, characterized by means for repeating said registering step, meansfor storing said registered pressure functions, and means for evaluatingtrends in said switching-related information.
 15. The means according toclaim 14, characterized by means for triggering an alarm signal if anincreasing or decreasing trend in said switching-related information isdetected.
 16. A tap changer having an insulating liquid and comprisingmeans for performing a switching action of said tap changer anddiagnosing means comprising means for registering pressure in saidinsulating liquid as a function of time for a period of time comprisingthe time in which said switching action is carried through,characterized in that said diagnosing means further comprises means forextracting switching-related information from said registered pressurefunction comprising means for identifying a first pressure peak in saidregistered pressure function; means for relating quantities from saidfirst pressure peak to quantities of a first arcing of said switchingaction; means for relating said quantities of said first arcing tobreaking a first contact of a diverter switch in said tap changer duringsaid switching action; and means for diagnosing a switching conditionassociated with said first contact, the switching condition being anerror condition when quantities are outside a predetermined interval.17. A transformer with a tap changer having an insulating liquid andcomprising means for performing a switching action of said tap changerand diagnosing means comprising means for registering pressure in saidinsulating liquid as a function of time for a period of time comprisingthe time in which said switching action is carried through,characterized in that said diagnosing means further comprises means forextracting switching-related information from said registered pressurefunction comprising means for identifying a first pressure peak in saidregistered pressure function; means for relating quantities from saidfirst pressure peak to quantities of a first arcing of said switchingaction; means for relating said quantities of said first arcing tobreaking a first contact of a diverter switch in said tap changer duringsaid switching action; and means for diagnosing a switching conditionassociated with said first contact, the switching condition being anerror condition when quantities are outside a predetermined interval.18. A computer program product comprising computer code means and/orsoftware code portions for making a processor perform the steps ofclaim
 1. 19. A computer program product according to claim 18 suppliedvia a network, such as Internet.
 20. A computer readable mediumcontaining a computer program product according to claim 18.